Man, if I had a nickle for every time someone has told me "We bought a bunny from the pet shop, but it died!" The pet shops want them at 5 weeks, because they still look like babies at that age. At 8 weeks, they look more like young rabbits, and the aw-w-w-w cute factor is no longer in play. I will "rent" bunnies at this age for photo shoots or Easter egg hunts, but it's only for a couple of hours, then they come back home to the old familiar cage and Momma. In the twenty years plus that I have been raising rabbits, I haven't lost a bunny yet this way (fingers crossed!)
I will not sell bunnies at this age, for the very reason that LowriseMinis gave. While it is true that commercial breeders will wean bunnies at 4 weeks, they aren't moving them to a completely new setting. Often, they will move the doe to a new cage, and everything else for the bunnies remains exactly the same. Also, they expect to lose a few bunnies in the process of growing them up. To me, any bunny losses after the first two weeks are unacceptable.
I don't mean to be quarrelsome, joyenes, but I have seen bunnies nursing well beyond 4 weeks of age. I have some silly does that will continue to nurse babies that are nearly as big as they are, if the bunnies are still in the cage with them. Pet breed does aren't usually worked as hard as commercial does; breeders that are focusing on show quality bunnies seldom breed for more than 4 or 5 litters from the same doe in a year.
If I have bunnies that have been orphaned before 6 weeks of age, or for any reason seem to not be doing as well as I would like them to be, I will add oatmeal to their pellets. Plain, dry rolled oats are a good baby food. Also, if a bunny's poop is a bit gooey, oatmeal can help to firm it up. I agree with what the others have said regarding feeding, that's the only thing I would add.